In recent years, photoinitiators are gaining in importance in the field of UV-light and daylight curable resin compositions, such as photocurable coatings or photocurable printing inks, which may be applicable to a wide range of substrates including metal, paper, plastics and wood, since the photoinitiators can drastically increase curing rate.
In this field, highly sensitive photoinitiators are desired, which are capable of absorbing enough UV or daylight, and producing radicals that start polymerization or transferring the absorbed energy to polymerizable substances for formation of radicals.
Conventionally, photoinitiators have been repeatedly studied so far with the aim of enhancing sensitivity, and improving inherent technical problems such as yellowing. In order to overcome these general problems, for example, alkylphenylbisacrylphosphine oxides, and photoinitiator mixtures of an alkylphenylbisacrylphosphine oxide with benzophenone are disclosed in U.S. Pat. No. 6,020,528.
The conventional photoinitiators, such as alkylphenylbisacylphosphine oxides exhibit a good curability, particularly when used in thin layer photocurable coatings. However, the photoinitiators are difficult to use for UV curing of coatings having a greater layer thickness. This is because the self-absorption of the photoinitiator molecule containing aromatic structure elements is large.
Furthermore, the photoinitiators of the alkylphenylbisacylphosphine oxide type exhibit an improved yellowing behavior in comparison with other conventional photoinitiators (e.g., benzophenone). However, the problem of yellowing has not been completely solved thus far. Therefore, there is a strong need for a photoinitiator showing significant improvement regarding yellowing.